An olfactory receptor is a trimeric G protein-coupled receptor (hereinafter, referred to as “GPCR”). More particularly, the olfactory receptor is one kind of trimeric G protein-coupled seven-transmembrane receptor.
FIG. 1 shows a mechanism by which a stimulus of an odor molecule to a cell membrane is converted into an electric signal.
The olfactory receptor is a membrane protein which is expressed on the cell membrane. The cell membrane is mainly composed of a lipid bilayer membrane. The lipid bilayer membrane has a structure of two layers each consisting of phospholipid molecules arranged in a high density. This lipid bilayer membrane is shown in the center of FIG. 1 schematically. In FIG. 1, the outside of the cell is above the upper part of the lipid bilayer membrane. On the other hand, the inside of the cell is below the lower part of the lipid bilayer membrane. The trimeric G protein is placed in the vicinity of the olfactory receptor.
The trimeric G protein is a heterotrimer comprised of an alpha subunit (Gαolf), a beta-subunit (Gβ), and a gamma subunit (Gγ). The cell contains adenylate cyclase. In FIG. 1, the adenylate cyclase is referred to as “AC”. To be more exact, the adenylate cyclase is a transmembrane-type protein. A protein RTP1S (SEQ ID NO: 42) has a function to assist the olfactory receptors to be expressed in the cell membrane. It is noted that the protein RTP1S is not directly associated with the mechanism.
Next, the mechanism is described. The odor molecule binds to the olfactory receptor. The binding separates the trimeric G protein into the alpha subunit (Gαolf) and a beta-gamma complex. The beta-gamma complex consists of the subunit Gβ and the subunit Gγ. The separated Gαolf activates the adenylate cyclase (AC). The activated adenylate cyclase (AC) converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP).
The cyclic adenosine monophosphate (cAMP) activates an ion channel, more particularly, for example, a cyclic nucleotide gated ion channel (CNG). The activation allows an ion to be transported from the inside of the cell to the outside of the cell, or from the outside of the cell to the inside of the cell. The degree of the transport of the ion can be measured as an electric signal.
Needless to say, when the production amount of the cyclic adenosine monophosphate (cAMP) is greater, an amount of the obtained electric signal is greater. The greater amount of the electric signal improves the measurement accuracy.
Further, it is known that a stimulus to a beta-1 adrenergic receptor increases the production amount of the cAMP in the cell. When many olfactory receptors in which a part of the beta-1 adrenergic receptor is incorporated are expected to increase the product amount of the cAMP and to improve the measurement accuracy thereby. Such an olfactory receptor, namely, the olfactory receptor in which a part of a different receptor (in this case, beta-1 adrenergic receptor) is incorporated, is referred to as a “chimeric olfactory receptor”.